The long circuit setup times associated with known telephone switching systems, e.g., crossbar systems, make such systems impractical for applications requiring frequent, short-duration, data communications. Circuits are typically established in such systems by a central control complex only after an available network path is found by hunting through a large centralized database. Not only is such a path hunt slow, but many additional communications are required both to request circuits and to keep the centralized database informed of every network status change.
One known approach to the problem of switching packetized information is disclosed in U.S. Pat. No. 4,524,440, issued to M. Orsic. The system is referred to as a fast circuit switching system, since a separate circuit is established for each packet-sized data communication. Information is conveyed from a number of communications modules in source channels to a number of port controllers and to a network. Information is conveyed from the network to the communications modules in destination channels. Each communications module includes a transmitter that transmits circuit setup request signals defining requested destination channels. Each port controller stores one of a number of status words defining both the availability of the destination channels and the availability of associated receivers included in the communications modules. Each of these status words is cycled to each port controller. When one of the status words cycled to a port controller defines that a requested destination channel and an associated communications module receiver are available, the port controller transmits a circuit setup request signal and subsequent data to the network. The network responds to the circuit setup request signal by establishing a circuit to the requested destination channel. The port controllers of the system use the cycled status words to advantage to control the transmission of packets by the communications modules. However, substantial circuit setup delays can occur because of the time required to cycle the status words to the port controllers. Such delays result in a significant waste of the available network bandwidth to the destinations. For example, when the status for a particular destination has just been cycled from a given port controller, the transmission of a packet to that destination is delayed until the status word is cycled through all other port controllers and returned again to the given port controller. The delay occurs even though none of the other communications modules are contending to transmit a packet to that destination. The status word cycle time also delays the updating of status words to reflect changes in receiver availability. Not only can this unnecessarily delay transmission of a packet to a receiver that has just become available, but it also means that a packet may be transmitted to an unavailable receiver before the status word is updated to reflect the unavailable status. In the latter case, the packet may be lost. In applications where reliable packet communication is required, relatively complex packet protocols and additional packet buffering are typically implemented to allow retransmission of lost packets.
In view of the foregoing, a recognized problem in the art is the delay in enabling packet transmission in known switching arrangements. Not only does the delay waste available system bandwidth, it may also necessitate the use of complex packet processing techniques to assure reliable communication.